Seat-Of-The-Pants Meaning

(chiefly US, idiomatic) Done by feel, guess, or trial and error rather than by careful planning, thought or technique.

Example: 1984 Jan. 9, Charles Alexander and Frederick Ungeheuer, "The Billion-Dollar Boys," Time:
  The new pension-fund managers are an eclectic group, displaying a spectrum of styles and strategies. Some rely chiefly on computers and charts, while others are more likely to trust their seat-of-the-pants judgment.